1. Field of the Invention
This invention relates to nonvolatile memory, and in particular to sense amplifier timing for reading nonvolatile memory.
2. Description of Related Art
Measurements of nonvolatile memory typically rely on a sense amplifier to compare a voltage or current from the nonvolatile memory against a reference voltage or current. A read reference level will typically be positioned to keep the sensing window for an inter-logical level. For example, if a lower range of threshold voltages represents a first logical value, such as “1”, and an upper range of threshold voltages represents a second logical value, such as “0”, then the read reference level might be positioned in the widow between those the upper and lower ranges. However, a refresh reference level might be positioned between the read reference level and the minimum of the upper range; and another refresh reference level might be positioned between the read reference level and the maximum of the lower range. A disagreement between measurements using the read reference level and the refresh reference level indicates the need to refresh a memory cell which is supposed to store a logical value corresponding to the upper range or the lower range. A program verify reference level might be positioned at the minimum of the upper range. An erase verify reference level might be positioned at the maximum of the lower range.
Additionally, if a nonvolatile memory cell location stores multiple bits, then more than two threshold voltage ranges exist. Consequently, a nonvolatile memory cell characterized by multi-level storage has a read reference level, a refresh reference level, a program verify level, and an erase verify level for every inter-logical level window, between any two neighboring threshold voltage ranges.
The existence of several different reference levels, varying with threshold measurement type (e.g., threshold window sensitivity and inter-logical level window), complicates sense amplifier circuitry design, as reference circuitry specific to each reference level must be connected to the sense amplifier circuitry.
Therefore, it would be desirable to simplify the circuitry that generates the various reference levels for sense amplifier circuitry.